Medication is often dispensed using a medicine cartridge, such as a glass syringe, having a barrel with a needle at one end and a plunger slidably inserted into the other end and coupled to a rubber stopper. Such cartridges are often referred to as “pre-filled syringes” because they may contain a specific dosage or volume medication when they are initially provided, as compared to conventional syringes that are furnished empty and filled by the user before making an injection.
The glass syringe and rubber stopper have, for years, provided an ideal drug storage closure having unique properties of impermeability to oxygen, low extractables, biocompability, durability, etc. However, they are both formed by processes that do not lend themselves to tight geometrical tolerances. Tight tolerances were not originally needed by these devices because they were not used mechanically with other devices.
Due to the risk of communicable diseases, a number of syringes and adapters have been developed that are intended to prevent accidental needle sticks and/or inadvertent reuse of a syringe. Conventional passive anti-needle stick safety devices for prefilled syringes must mount to the syringe but not interfere excessively with the force required to move the plunger rod during injection nor prevent the full travel of the plunger rod. The safety mechanism necessarily must be triggered toward the end of administration of the drug (near the end of the plunger rod travel).
In some instances of conventional devices, the distance between trigger fingers of such a safety device is less than the diameter of a standard medicine cartridge flange. Consequently, during insertion of the medicine cartridge subassembly into the safety device, the trigger fingers must be flexed outwardly, which causes unseating of the trigger fingers from the body of the safety device for a time until the medicine cartridge is sufficiently inserted into the safety device. This can lead to a partially activated device, which is described as one trigger finger seated and one trigger finger unseated. Furthermore, a serious problem with specific safety devices is the significant force required to insert the medicine cartridge into the safety device. The significant force puts a large amount of stress on the medicine cartridge flange, typically made of brittle material, which greatly increases the risk of breakage.
Prefilled single-dose glass syringes were originally designed to be used as a manually operated device. To facilitate this, a finger flange is typically formed at the proximal end of the syringe barrel by heating the glass and splaying the cylinder walls externally to form a flange. Two opposite sections of the resultant round flange were sometimes cut when the glass was still molten so as to produce a flange with more diametrically opposed surfaces suitable for being grasped and supported by the user's first two fingers. These operations commonly impose internal stresses in the glass around the flange area.
In order for the syringe to be integrated with these safety devices, it must be installed and rigidly held in place by some feature so that the syringe can resist displacement during needle insertion and medication dispensing. The syringe finger flange is the usual feature with which the auxiliary devices grasp and hold the syringe in position, however this is also the feature that sustains internal stresses during the manufacturing operations. As a result, it is common for syringes to break during the assembly operations with auxiliary devices causing the spread of glass fragments into surrounding product and machinery. This in turn causes assembly line stoppage, cleanup efforts, and line clearance (waste) or re-inspection of product. This problem has recently prompted syringe manufacturers to develop syringes with smaller round flanges (e.g. smaller overall diameter) so that the glass manufacturing methods impose fewer internal stresses. Additionally, the smaller exposed geometry of these flanges prevents the auxiliary devices from imposing large stresses to the flange during assembly and functional use.
However, these smaller flanges present less geometry with which the auxiliary devices can hold the syringe. Since most auxiliary devices are made of plastic using an injection molding process, the holding means for the syringe are commonly flexible elements that are integral to a main component of the device
This avoids using a secondary component to secure the syringe, which would require additional manufacturing steps. As the syringe is inserted, these flexible elements displace away from the flange under the force of insertion, allowing the syringe to move up to a reference surface. Once the flange has reached the reference surface, the flexible elements substantially return to their previous position and thereby provide a force-limited retention of the syringe to the device. Because large auxiliary devices are not desirable, the flexible elements are usually small, which inherently limits how much they can elastically flex. Because the edge of the flange is round due to the molten state of the glass, etc. during forming, a large portion of the projected geometry with the small flange is not suitable for resisting an axial load on the syringe as would be generated in normal use of the device. Therefore, it is desirable to provide a means for increasing the holding strength of retention features for use with syringes having smaller flange diameters.
Accordingly, an improved needle guard for a medicine cartridge or syringe is desirable.